Assessment of porosity index of the femoral neck and tibia by 3D ultra‐short echo‐time MRI

Purpose To measure the porosity index (PI) by ultrashort echo‐time magnetic resonance imaging (UTE MRI) of the femoral neck and tibia; assess its correlations with age, gender, and body mass index (BMI); and analyze the PI correlations between both sites to assess whether tibial PI can reflect changes of femoral neck PI. Materials and Methods In all, 68 healthy men and women (mean age, 45.7 ± 15.9 years) underwent 3D UTE MRI (3.0T) of the hip and mid‐shaft tibia. PI of the inferior femoral neck cortex and the whole cortex of the tibia were analyzed. Associations between parameters and differences of PIs between men and women, pre‐ and postmenopausal women were tested. Results Femoral neck PI was negatively correlated with age ( r = –0.385, P = 0.043) and curvilinearly correlated with BMI (R 2 = 0.225, P = 0.041) in men. Tibial PI was correlated with BMI ( r = –0.477, P = 0.002) in women and age ( r = 0.469, P = 0.043) in postmenopausal women, although P = 0.097 ( r = 0.403) after adjustment for BMI. Femoral PI was significantly higher in men than in women ( P < 0.001). No significant difference in femoral and tibial PI was observed between pre‐ and postmenopausal women. The femoral neck and tibial PIs were not significantly correlated in any group. Conclusion PIs had some correlations with age, gender, and BMI. Since femoral neck PI was not correlated with tibial PI, the tibia cannot substitute the femoral neck for estimating bone quality. Direct assessment of the femoral neck cortex is needed. Level of Evidence : 3 Technical Efficacy Stage : 3 J. Magn. Reson. Imaging 2018;47:820–828.